The present invention relates generally to fabrication of integrated circuits, and more particularly, to a method for deposition of in-situ doped amorphous semiconductor film on a first set of semiconductor wafers and for deposition of undoped semiconductor film on a second set of semiconductor wafers within a same reaction chamber.
Doped amorphous semiconductor film is used in integrated circuits for various purposes such as for forming a gate structure of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The doped amorphous semiconductor film, such as doped amorphous silicon for example, in the prior art is typically formed by first forming a semiconductor film that is substantially undoped and then implanting dopant into the semiconductor film to enhance the conductivity of the semiconductor film. The dopant that has been implanted into the semiconductor film is then activated in a thermal anneal process to further enhance the conductivity of the semiconductor film from activation of the dopant within the semiconductor film, as known to one of ordinary skill in the art of integrate circuit fabrication.
However, such an implantation process is disadvantageous because at least three steps are used for forming the doped semiconductor film including formation of the undoped semiconductor film, implantation of the dopant into the semiconductor film, and then a thermal anneal process for activation of the dopant within the semiconductor film. Each additional step during integrated circuit fabrication introduces added cost and delay to the manufacture of integrated circuits.
A chemical reaction process such as a low pressure chemical vapor deposition process is used for formation of a doped semiconductor film with in-situ doping of the semiconductor film. With xe2x80x9cin-situxe2x80x9d doping, dopant is incorporated into the semiconductor film simultaneously during deposition of the semiconductor film. Thus, a chemical reaction process for formation of the semiconductor film with xe2x80x9cin-situxe2x80x9d doping avoids the three steps of the dopant implantation process.
Nevertheless, a chemical reaction process of the prior art is disadvantageous because defects may tend to form more easily in such a chemical reaction process than in an implantation process. Furthermore, electrical characteristics such as the sheet resistance of the semiconductor film is typically harder to control in a chemical reaction process than in an implantation process. In addition, the uniformity of electrical characteristics such as the sheet resistance of the semiconductor film across the semiconductor wafer is also typically harder to control in a chemical reaction process than in an implantation process.
For avoiding the three steps of the implantation process, an improved chemical reaction process is desired for formation of a semiconductor film with in-situ doping of the semiconductor film and with reduced defects and with predictable electrical characteristics such as controllable and uniform sheet resistance across the semiconductor wafer. Thus, the earlier filed copending patent application, with Ser. No. 09/521,591 describes a method and apparatus for depositing an in-situ doped amorphous semiconductor film within a reaction chamber to avoid the three steps of the implantation process.
After such an in-situ doped amorphous semiconductor film is deposited on a first set of semiconductor wafers in the reaction chamber as described in Ser. No. 09/521,591, a second set of semiconductor wafers may require deposition thereon of an undoped semiconductor film, such as polysilicon for example. In the prior art, a different reaction chamber is typically used for deposition of the undoped semiconductor film on the second set of semiconductor wafers. However, another reaction chamber adds cost to integrated circuit manufacture. In addition, another reaction chamber typically requires added labor for maintenance and monitoring of operation.
Thus, a process is desired for depositing in-situ doped semiconductor film, such as doped amorphous silicon for example, on a first set of semiconductor wafers and for depositing undoped semiconductor film, such as undoped polysilicon for example, on a second set of semiconductor wafers, within a same reaction chamber.
Accordingly, in a general aspect of the present invention, a coating of a first undoped semiconductor film is deposited on the wafer boat and on components of the reaction chamber after in-situ doped amorphous semiconductor film has been deposited on a first set of semiconductor wafers and before undoped semiconductor film is deposited on a second set of semiconductor wafers within the same reaction chamber.
In one embodiment of the present invention, in a method for depositing semiconductor films on a plurality of sets of semiconductor wafers, a first set of semiconductor wafers carried by a wafer boat are placed within a reaction chamber. An in-situ doped amorphous semiconductor film is deposited on the first set of semiconductor wafers while the first set of semiconductor wafers carried by the wafer boat are within the reaction chamber. The first set of semiconductor wafers carried by the wafer boat are removed from the reaction chamber, and the first set of semiconductor wafers are removed from the wafer boat. The wafer boat that is empty of any semiconductor wafers is placed back within the reaction chamber. A first undoped semiconductor film having a thickness in a range of from about 8,000 xc3x85 (angstroms) to about 12,000 xc3x85 (angstroms) is deposited on the wafer boat and on components of the reaction chamber. The wafer boat is then removed from the reaction chamber, and a second set of semiconductor wafers are loaded within the wafer boat. The wafer boat having the second set of semiconductor wafers loaded therein is placed within the reaction chamber. A second undoped semiconductor film is deposited on the second set of semiconductor wafers while the second set of semiconductor wafers carried by the wafer boat are within the reaction chamber.
In one embodiment of the present invention, for depositing the first undoped semiconductor film on the wafer boat and the components of the reaction chamber, the pressure within the reaction chamber is set to be about 0.19 Torr. The temperature within the reaction chamber is set to a predetermined temperature in a range of from about 600xc2x0 Celsius to about 650xc2x0 Celsius. A semiconductor film reactant flows through at least two gas inlets. Each gas inlet is disposed on a respective location of the reaction chamber near the wafer boat, and each gas inlet carries the semiconductor film reactant. The first undoped semiconductor film is formed on the wafer boat and the components of the reaction chamber from the semiconductor film reactant in a LPCVD (Low Pressure Chemical Vapor Deposition) process within the reaction chamber.
The present invention may be used to particular advantage when the in-situ doped amorphous semiconductor film deposited on the first set of semiconductor wafers is comprised of phosphorous doped amorphous silicon and the first undoped semiconductor film is comprised of undoped polysilicon with the semiconductor film reactant being comprised of silane (SiH4). In that case, the second undoped semiconductor film deposited on the second set of semiconductor wafers may also be comprised of undoped polysilicon film.
In this manner, the relatively thick first undoped semiconductor film encapsulates any dopant remaining from the prior deposition of the in-situ doped amorphous semiconductor film before the second set of semiconductor wafers are placed into the wafer boat and the reaction chamber for deposition of the second undoped semiconductor film on the second set of semiconductor wafers. The in-situ phosphorous doped amorphous silicon film is deposited on the first set of semiconductor wafers and the undoped polysilicon film is deposited on the second set of semiconductor wafers within the same reaction chamber to minimize cost and labor during integrated circuit manufacture.
Furthermore, for reducing defects on the semiconductor wafers, a vacuum suction may be applied through the at least two gas inlets and nitrogen (N2) gas may be purged through the at least two gas inlets, once before deposition of the coating of the first undoped semiconductor film on the wafer boat and on the components of the reaction chamber, and alternatingly at least three times after deposition of the coating of the first undoped semiconductor film on the wafer boat and on the components of the reaction chamber. This purging reduces the defects by reducing the number of contaminant particles within the reaction chamber.
These and other features and advantages of the present invention will be better understood by considering the following detailed description of the invention which is presented with the attached drawings.